"Theme development is a tedious and difficult task, and for the GTK devs to be so careless in breaking their API at every turn disrespects the many hours people put into making themes for it. [...] I was given to believe that this breakage stems from a Microsoft-like climate of preventing users from customizing their systems, and deliberately breaking the work of others so that your 'brand' is the best. Anytime I hear the word 'brand' being used in Linux, I know something valuable is being poisoned." I find the tone of this one a bit too harsh and overly negative at times, but his point still stands.

Easy solution to this: Some developers from XFCE and/or LXDE contribute some man power to maintaining GTK?

No because I think everyone outside of the Gnome/GTK projects now knows not to deal with Gnome/GTK development. Anyone who has tried in the past has been met with a campaign of misdirection and misinformation as well as being marginalised via Gnome/GTK devs going 'radio silent'.